A stepper-type or step-and-scan-type projection aligner is used in the manufacture of semiconductor devices and the like to transfer a reticle pattern as a photo mask to each shot region on a wafer coated with a photoresist through a projection optical system.
Resolution of a projection optical system used in a projection aligner increases as the exposure wavelength decreases and the numerical aperture of the projection optical system increases. Therefore, the exposure wavelength which is a wavelength of radiation used in the projection aligner has been reduced in accordance with scaling down of integrated circuits year by year, and the numerical aperture of the projection optical system has been increased.
Depth of focus is as important as resolution when a resist is exposed to radiation. The resolution (R) and the depth of focus (δ) are shown by the following formulas:R=k1·λ/NA  (i)δ=k2·λ/NA2  (ii)wherein λ is the exposure wavelength, NA is the numerical aperture of the projection optical system, and k1 and k2 are process coefficients. When obtaining the same resolution R, a larger depth of focus δ is obtained by using radiation with a shorter wavelength.
In this instance, a photoresist film is formed on the surface of the exposed wafer, and a pattern is transferred onto this photoresist film. In a generally-used projection aligner, the space in which the wafer is disposed is filled with air or nitrogen. When the space between the wafer and the lens of the projection aligner is filled with a medium having a refractive index of n, the resolution R and the depth of focus δ are shown by the following formulas.R=k1·(λ/n)/NA  (iii)δ=k2·n λ/NA2  (iv)
For example, when water is used as the above medium in an ArF process, the resolution R is 69.4% (R=k1(λ/44)/NA) and the depth of focus is 144% (δ=k2·1.44 λ/NA2) with respect to in which the photoresist is exposed through the air or nitrogen, when the refractive index of light with a wavelength of 193 nm in water is n=1.44.
Such a projection exposure method in which the wavelength of exposure radiation is reduced to transfer a more minute pattern is called liquid immersion lithography. The liquid immersion lithography is considered to be an essential technology for lithography with reduced dimensions, particularly for lithography with dimensions of several tens of nanometers. A projection aligner used for the method is known (see Patent Document 1).
In the liquid immersion lithographic method using water as a medium of immersion, a photoresist film formed on a wafer and the lens of a projection aligner are brought into contact with water. For this reason, water may permeate the photoresist film and decrease the resolution. In addition, the photoresist may elute its components into water, and the water may pollute the surface of the lens of a projection aligner.
A method of forming an upper layer film on a photoresist film may be used in order to block the photoresist film from the medium such as water. Such an upper layer film must possess sufficient transparency to radiation of a wavelength used in liquid immersion lithography, must form a protective film on a photoresist film while causing almost no intermixing with the photoresist film, must be stably maintained without being dissolved in the medium such as water during liquid immersion lithography, and must be easily dissolved in a developer such as an alkaline solution.
Although the resist pattern forming methods described in Patent Documents 2 and 3 can suppress a watermark defect which is a phenomenon of leaving scars of immersion liquid droplets on a resist pattern caused by an immersion liquid penetrating a protective film and remaining therein and a pattern defect which is an irregular pattern in which the line width is large in some areas, but small in other areas, these methods have another problem of a blob defect which is caused by reattachment during a rinsing operation. In other words, although a resist pattern with high resolution can be expected while effectively suppressing the watermark defect and the pattern defect when using the resist pattern forming methods described in Patent Documents 2 and 3, these methods are not sufficient because of occurrence of the blob defect. Therefore, further improvement has been desired.    Patent Document 1: JP 11-176727 A    Patent Document 2: JP 2005-264131 A    Patent Document 3: JP 2006-64711 A